UK Antarctic Meteorite Classification – the process

The first of the UK Antarctic meteorites have been classified! Nine rocks retrieved from the Hutchison (HUT) and Outer Recovery (OUT) icefields have been approved and published by the Meteoritical Bulletin. For details of these, please see this table and follow the links.

How were they classified?
The rocks collected in Antarctica were shipped frozen to the UK and kept in temperature monitored freezers. Each rock was thawed in an exisccator chamber to minimise any chemical reaction with residual ice; any ice present should sublimate straight to gas from the solid phase. After thawing, the rocks were all individually weighed and photographed. Measurements were taken of the rock’s magnetic susceptibility and electrical conductivity (see this previous blog post by Tom Harvey, as these properties can give a provisional indication of their possible classification.

Some of the meteorites were chosen to undergo computerised tomography (CT) scanning and photogrammetry. CT-scanning gives a 3D image of the interior of the rock. This provided compositional information and helped us to choose where to attempt to break or cut the rock. Photogrammetry allows a full 3D surface rendering of the meteorite to be made. For more details of this work, please see Tom’s LPSC conference presentation.

20200130_103524
Tom Harvey in the curation lab imaging one of the meteorite samples. Photo: KJoy

Where there were no natural crumbs or fragments of material for further analysis, each rock was split with rock splitters, keeping them inside their sterile bag so that the only contact was with the stainless steel blade. A few meteorites and some of the fragments from the rock splitters were cut with a low speed saw to ensure the material does not get hot enough to cause any alteration.

Fragments were weighed, photographed and then made into epoxy blocks. These have to be polished to a very flat surface, in order to be able to examine the chemical composition using a scanning electron microscope (imaging system) and electron microprobe (used to determine mineral chemistry).

The scanning electron microprobe provides greyscale images (see left below), where the brightness corresponds to the differences in atomic number (number of protons) of the different elements. For example, iron, with an atomic number of 26, will be much brighter compared to sodium, with an atomic number of 11. It can also be used to quantify the difference in this brightness to get an idea of the relative abundance of each element and likely mineral. We use the scanning electron microscope first to get an image and element maps of the whole section (see right image below).

Following this, around 30-40 spots (points) are selected for measurement with the electron microprobe as this allows for more precise and accurate quantification of the mineral composition. In the element map above, the silicate minerals shown in green are usually olivine, and the minerals in light blue are usually pyroxene. The iron content of olivine and pyroxene varies and this is used to distinguish ordinary chondrites between the “H”, “L”, and “LL” classifications (check out where these meteorites sit within the family tree here).  “H” chondrites have the highest iron metal content, but lower iron oxide in pyroxene and olivine. “L” chondrites have lower iron and less metal but higher iron oxide in pyroxene and olivine, whereas “LL” have the lowest total iron but the highest iron oxide in pyroxene and olivine.

Graph showing the iron oxide abundance in pyroxene (Fs) against the iron oxide abundance in olivine (Fa). Yellow crosses show the data collected for five UK Antarctic meteorites, superimposed on a graph of values recommended as distinguishing the H, L and LL ordinary chondrite groups for the Nomenclature committee (Grossman & Rubin, 2006).

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